This invention relates to an optical disk drive apparatus with high-speed access performance.
Optical disks have recently come into use as data storage media in computer systems, and disk drive apparatus has been developed for reading and writing such disks. A block diagram of the control system of an optical disk drive apparatus similar to that described in Japanese Patent Application Laid open No.156526/1986 is shown in FIG. 6. In this apparatus, information is written on or read from concentric circular or spiral tracks on an optical disk 1, shown here in cross section. The information is conveyed to or from the optical disk by a light beam 2 produced by an optical head 3. The light beam 2 is focused by a lens onto a spot on the optical disk 1. The optical head 3 is mounted on a carriage 4 which can be driven by a linear actuator 5 to move the spot of the light beam 2 from one track to another. When the spot is following a track, a tracking actuator 6 turns the lens to keep the spot positioned at the center of the track. A tracking sensor 7, comprising a pair of photosensitive elements, senses the light reflected from the disk surface. The electronics of the apparatus comprise a difference amplifier 11 and summing amplifier 12 that process the signals from the photosensitive elements, a speed detection circuit 13 and direction detection circuit 14 that detect the motion of the spot, a spot velocity detection circuit 15, a pulse generator circuit 16 that generates one pulse for each track crossed by the spot, a track counter 17 that counts these pulses, a reference velocity generating circuit 19, a velocity error detection circuit 21, an amplifier circuit 22, a tracking command circuit 25, and a tracking servo circuit 26. The operation of the control system is described next.
The control system operates in two modes: a track following mode, the purpose of which is to keep the spot of the light beam 2 centered on the current track; and a track access mode, the purpose of which is to move the light beam 2 from the current track to a target track. Normally the control system stays in the track following mode. A transition to the access mode occurs when a command circuit 90 supplies an access command to move to a new target track. The target track is specified by two input signals: a stroke count input (N) indicating the number of tracks the spot of the light beam 2 must move to get from the current track to the target track; and a direction input (D) indicating whether the light beam 2 must move toward the center or the periphery of the disk. The stroke count input is received by the track counter 17 and presets the track counter 17 to the value of N. The output of the track counter 17 is sent to the reference speed generating circuit 18. When the reference speed generating circuit 18 receives the access start command S14 and the initial count signal N, it generates and stores a reference speed pattern indicating how the speed to the spot should vary during the access operation. It supplies the reference velocity generating circuit 19 with a speed signal of this pattern which varies with the count value from the track counter 17 indicating the diminishing number of remaining tracks.
The reference velocity generating circuit 19 receives in addition to the reference speed signal, the direction input D indicating the direction in which the light beam 2 should move. The reference velocity generating circuit 19 combines this speed and direction information into a reference velocity signal, which it sends to the velocity error detection circuit 21. The velocity error detection circuit 21 compares the reference velocity with the current velocity of the spot and generates a velocity error signal. The amplifier circuit 22 amplifies this velocity error signal and controls the linear actuator 5 in such a ay as to reduce the velocity error to zero. The linear actuator thus drives the carriage 4 so that the spot of the light beam 2 is forced to move toward the target track at a velocity matching the reference velocity.
As the spot of the light beam 2 crosses the tracks, the intensity of the reflected light varies. The pair of photosensitive elements in the tracking sensor 7 receives the reflected light and converts it to a pair electrical signals that vary cyclically, due to variation in the reflected light, with a frequency proportional to the speed of the spot. Due to the placement of the photosensitive elements, these electrical signals are out of phase with each other, but are equal when the spot is either centered on a track or located at the midpoint between two tracks.
The electrical signals from the tracking sensor 7 are sent to the difference amplifier 11 and the summing amplifier 12, which obtain their difference and sum respectively. The speed detection circuit 13 receives the output from difference amplifier 11 and detects from its frequency the speed of the spot. The direction detection circuit receives he outputs from both the difference amplifier 11 and summing amplifier 12 and detects from their phase relationship the direction of motion of the spot. The spot velocity detection circuit 15 receives a speed signal from the speed detection circuit 13 and a direction signal from the direction detection circuit 14 and combines them into a signal indicating the current velocity with which the spot is moving on the optical disk 1. This signal is sent to the velocity error detection circuit 21, Which compares it with the reference velocity signal from the reference velocity generating circuit 19. The result is amplified by the amplifier circuit 22 and controls the linear actuator as described earlier, causing the light beam 2 to move toward the target track at the velocity specified by the reference velocity signal.
The output from the difference amplifier 11 is also sent to the pulse generator circuit 16. This output varies cyclically above and below zero with the zero point occurring when the light beam is centered on a track or at the midpoint between two tracks. The pulse generator circuit 16 generates a pulse that is High hen the difference signal is negative and Low when the difference signal is positive. One pulse is thus generated per complete cycle of the difference signal, hence there is one pulse per track crossed by the spot of the light beam 2. The track counter 17 receives these pulses and decrements by one count for each pulse received. Since the initial count is the stroke count N, at any given point during the access operation the count output by the track counter 17 indicates the number (OA) of remaining tracks the spot of the light beam 2 has to go to reach the target track. As it receives the diminishing counts from the track counter, the reference speed generating circuit 18 sends the corresponding reference speed signals from its stored pattern to the reference velocity generating circuit 19. The spot thus moves according to the pattern, first accelerating, then traveling at a steady speed, then decelerating as it approaches the target track.
The tracking command circuit 25 receives the count output OA from the track counter 17 and the speed signal from the speed detection circuit 13. When it receives a zero count signal, indicating that the spot is on or almost on the target track, it waits for the spot speed to be reduced to a sufficiently low level, then commands the tracking servo circuit 26 to begin driving the tracking actuator 6. This marks the return from the track access mode to the track following mode. In the track following mode the tracking servo circuit 26, activated by the tracking command from the tracking command circuit 25, monitors the difference output signal from the difference amplifier 11 and controls the tracking actuator 6 so as to reduce the difference to zero; that is, to move the beam spot to the center of the track and hold it there. Information is then written or read as the light beam 2 follows the target track.
FIG. 7 illustrates an output S1 from the difference amplifier 11, the resulting pulse output S2 from the pulse generator circuit 16, and the count output OA from the track counter 17 When the control system operates correctly. The count signal OA rises from O to N when the access command is received, then decrements in steps of one count per track as the spot moves toward the target track.
FIG. 8 illustrates a problem that tends to occur in optical disk drive control systems like the one just described, the problem being fluctuations that tend to occur around the zero level due for example to an external vibration or other disturbance. These fluctuations are particularly problematical because the light beam moves slowly immediately after the start of access. The fluctations lead to pulse jitter in the output S2 of the pulse generator circuit. As a result, the track counter miscounts the number of tracks and the light beam 2 fails to reach the target track. The access operation must then be repeated until the target track is reached successfully. Such repetitions retard the access performance of the optical disk drive apparatus.